1. Field of the Invention
This invention relates to roller-hearth furnaces.
2. The Prior Art
Conventional roller-hearth furnaces are generally used for the heat-treatment of workpieces where rapid heating is either not possible or not desirable. They cannot be used for heat-treatments where rapid heating of workpieces, such as plates or strips, to high temperatures is required. For this latter type of heat-treatment it is typically necessary to employ soaking-pit and similar furnaces in which rapid heating can take place.
Also, in many instances it is undesirable--from the viewpoint of the end results desired for the workpieces--to heat too rapidly. For such applications the relatively slow-heating roller-hearth furnaces have heretofore been used in which the differential between furnace heat and workpiece temperature is limited to not very high values at all points of the furnace interior, in contrast to other heat-treating furnaces where it is often immaterial for the qualities of the treated workpieces that substantial temperature differences exist between the workpieces and the furnace heat.
However, for a variety of reasons know to those conversant with the art it is desirable to be able to employ roller-hearth furnaces also for the rapid heating of workpieces. In fact, repeated attempts have been made to construct special high-performance roller-hearth furnaces for this purpose. These special furnaces are characterized in that substantial differences can develop locally between the furnace temperature and the workpiece temperature, especially in the region where the workpiece enters the furnace and are rapidly heated to high temperatures. These attempts have never been successful because the rollers of the hearth were unable to withstand the prevailing conditions and became damaged so rapidly that their reduced service life made the proposition uneconomical. Attempts at providing rollers of improved construction and/or materials have heretofore always failed.